Best gas flow purge system when solids are present. When used with displacer-type instruments, it reduces the available level transmitter span, particularly if a bottom connection to the measurement chamber is used.
With angled nozzles it is common practice to use a diptube as shown in Figure 11.17. This is usually omitted with horizontal nozzles.
Liquid purges are sometimes used with AP transmitters for difficult applications where there is otherwise a tendency to plug the impulse lines. For accurate, constant flow, a purge-flow regulator is mandatory, and the calibration procedure should allow for impulse-line pressure drop.
Liquid purges are also sometimes used with displacer-type transmitters. Such installations should be designed to avoid any pockets in which solids can accumulate. In addition, if any damping is used, either an external orifice or an internal chamber, the calibration should allow for the extra head due to the purge flow. As an example, some years ago we had a problem where our fluid mechanics experts recommended a purge velocity of 4 ft/sec through the damping orifice at the bottom of the displacer housing to defeat plugging. We chose 3.0 gpm of a liquid with a specific gravity of 0.8. The orifice was calculated to have a diameter of 0.55 inch. The level in the housing was then 9.0 inches higher than it would have been with no purge.
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